The dominant process currently used for processing siliceous phosphates is the Crago "Double Float" process. In this process, deslimed phosphate ore is subjected to sizing. Typical sizing involves using a hydrosizer to size the deslimed feed into coarse (16.times.35 mesh) and fine (35.times.150 mesh) fractions. In some more sophisticated operations, three fractions are produced (16.times.24, 24.times.35, and 35.times.150 mesh). The sized feed is first subjected to rougher flotation after conditioning at 70% or higher solids with fatty acid/fuel oil at pH about 9 for three minutes. A significant amount of silica (sands) is also floated in this step. The rougher concentrate goes through a dewatering cyclone, an acid scrubber, and a wash box to remove the reagents from phosphate surfaces. After rinsing, the feed is transported into flotation cells where amines (sometimes with diesel) are added, and the silica is floated at neutral pH. Since about 30-40% by weight of the sands in the flotation feed are floated twice, first by fatty acid and then by amines, the Crago process is inefficient in terms of collector efficiency. Indeed, theoretical fatty acid efficiency in a typical plant is merely 5%. The rest of the reagents are wasted primarily because of silica. The trends of declining grade of phosphate deposits and soaring prices for fatty acid do not favor the standard Crago process.
Despite its inefficiency, the Crago process has been used widely in the phosphate industry in the past. However, due to increasing amine costs and decreasing phosphorous content in currently-mined ore, there is a need in the art for new methods of economically and efficiently beneficiating siliceous phosphates.